1. Field of the Invention
The invention relates to a device for register correction and for compensating for distorted sheets in a feeder, more particularly, a pregripper in the feeder, of a sheet-fed rotary printing press, having a frame-fixed brace disposed on a pregripper shaft and located approximately centrally between end supports on a gripper bar, the gripper bar being supported rigid against torsion at opposite ends thereof on the pregripper shaft and being capable of executing swivelling movements in a sheet-feeding direction and in a direction opposite thereto, the frame-fixed brace serving for elastically bending the gripper bar in the sheet-feeding direction before the sheet is gripped by sheet grippers which are disposed side by side on the gripper bar and are swivellable about an axis of the pregripper shaft.
The foregoing state of the prior art is exemplified in the published German Patent Document DE 88 16 641 U1. FIG. 3 of this reference shows an embodiment wherein a cam roller is supported on a three-point lever which, in turn, is swivellably mounted on a support arm which is disposed approximately centrally between two end supports of a gripper bar and is also joined firmly to the gripper bar, on the one hand, and is braced against the support arm between the pivot bearing and the gripper bar, on the other hand. The cam is located on a ring surrounding the pregripper shaft and is adjustable in the angular position thereof by an adjusting member so that, during the swivelling motion of the pregripper prior to reaching the end position thereof wherein the sheet grippers engage the sheet at the leading edge thereof, the cam roller runs up onto the cam earlier or later and more or less, depending upon the adjustment of the cam, and the gripper bar bends at the middle thereof in the sheet feeding direction. The sheet is thereby deformed in the middle with respect to the ends thereof, in order to effect a register correction or tension equalization. In the heretofore known arrangement of this reference, the deflection of the cam roller is less than the desired bending of the gripper bar, so that the force originating from the cam and the cam roller must be greater than the force required to deform the gripper bar. This increases the sensitivity or vulnerability of the system to dynamic factors and requires a very sensitive adjusting mechanism.